The annealing of a metal strip such as a stainless steel strip may result in the formation of oxides on the surface of the metal strip. These oxides are comprised of, for example, iron, chromium, nickel, and other associated metal oxides, and are removed or reduced prior to utilization of the strip. The oxides of stainless steel, however, can be resistant to the common acid treatments. In addition, these oxides adhere tightly to the base metal, and thus may require mechanical scale cracking such as shot blasting, roll bending, or leveling of the steel strip or electrolytic and/or molten salt bath treatment prior to pickling (removal of the oxides on the surface of the strip) to either loosen these oxides or make the oxide surface more porous before pickling the strip.
Traditionally, the oxides on the surface of the stainless steel have been removed, or “pickled off”, using nitric acid in combination with hydrofluoric acid; or using a combination of hydrogen peroxide, sulfuric acid, and hydrofluoric acid, such as disclosed in U.S. Pat. No. 6,645,306, entitled Hydrogen Peroxide Pickling Scheme for Stainless Steel Grades,” issued Nov. 11, 2003, which patent is incorporated by reference herein. Such acids, particularly hydrofluoric acid, are expensive. Further, nitric acid is not considered environmentally friendly.
The present application describes a process for pickling stainless steel by preparing a mixture of an acid such as sulfuric acid (H2SO4), an excess of hydrogen peroxide (H2O2), and at least one electrode set including at least one of a cathode or anode and applying a current to a metal strip (such as a stainless steel strip) running through the mixture. Because of an excess of H2O2, all ferrous sulfate is converted to ferric sulfate (Fe2(SO4)3), which acts as an oxidizing agent itself. The process allows for a reduction of total chemicals consumed in the pickling process from known pickling processes and particularly for a reduction of nitric acid (HNO3) and/or hydrofluoric acid (HF) over known pickling processes. Further, certain ferritic stainless steels can be pickled without including HF in a pickling process utilizing the above disclosed mixture of an acid such as sulfuric acid (H2SO4), an excess of hydrogen peroxide (H2O2), and at least one electrode set.
The drawings are not intended to be limiting in any way, and it is contemplated that various embodiments of the invention may be carried out in a variety of other ways, including those not necessarily depicted in the drawings. The accompanying drawings incorporated in and forming a part of the specification illustrate several aspects of the present invention, and together with the description serve to explain the principles of the invention; it being understood, however, that this invention is not limited to the precise arrangements shown.